1. Field of the Invention
The present invention relates to photographic materials having an anti-halation layer easily removable from one surface of a substrate which carries a light-sensitive photographic emulsion layer on the other surface.
2. Description of the Prior Art
The phenomenon of halation takes place due to the fact that light reflected by the substrate of a photographic material again reaches the photo-sensitive emulsion coating; halation can thus be prevented by a light-absorbing anti-halation layer provided on the surface of the substrate other than that on which the photographic emulsion coating is provided.
Such an anti-halation layer generally comprises a natural or synthetic binder containing dyes or pigments.
Anti-halation layers can be classified into two types; one is referred to as a "decolorizable type" in which dyes included in the antihalation layer are decolorized or bleached during developing, and the other is referred to as a "removal type" in which the anti-halation layer itself is removed from the substrate during development.
For the latter type of anti-halation layer, i.e., the removable type, there are employed binders which are soluble in an aqueous alkaline solution. As such binder materials, extensive investigations have been performed, some of which are disclosed in U.S. Pat. No. 2,327,828 and Japanese patent application (laid-open) No. 2783/1971.
In a certain type of developing for color photography, a preliminary bath, which is strongly alkaline and contains a high concentration of an inorganic salt such as sodium sulfate, is used. The role of such a preliminary bath is not to remove the anti-halation layer from the photographic substrate but to impart water solubility to the binder for the anti-halation layer by converting the binder from its water-insoluble acid form into the corresponding alkali metal salt form.
Suitable aqueous alkaline solutions for preliminary baths include an aqueous solution of one or more inorganic salts adjusted to a pH value of about 9 to about 11. Typical inorganic salts used are sodium sulfate (Na.sub.2 SO.sub.4), sodium hydroxide (NaOH), borax (Na.sub.2 B.sub.4 O.sub.7) or mixtures thereof. The concentration of the inorganic salts is usually about 8 to about 15 wt%, preferably about 9.5 to about 12 wt%. A small amount of one or more organic salts such as sodium formate, sodium acetate, etc., may be added, if desired. The most preferable concentration of the aqueous alkaline solution is 8.5 to 9.5 wt% of sodium sulfate, 1.7 to 2.0 wt% of borax and 0.08 to 0.1 wt% of sodium hydroxide.
Usually, to prevent contamination of the preliminary bath (scum formation) the binder of the anti-halation layer is formulated so that it does not dissolve in the preliminary bath, which is strongly alkaline, even when it is present in the form of alkali metal salt, due to a salting-out effect caused by the presence of a high concentration of the inorganic salt. After leaving the preliminary bath, the anti-halation layer is stripped or removed off when the material is washed with water prior to first development.
From the standpoint of preventing scum formation or contamination, it is preferred that the anti-halation layer never be stripped from the substrate in the preliminary bath. However, insofar as one relies on an alkali soluble binder, the resulting anti-halation layer always exhibits insufficient film strength, making it difficult to avoid contamination of the preliminary bath.
After passing through the preliminary bath, the photographic substrate is washed with water, whereby the anti-halation layer is removed. In some instances, water jets are employed to perfectly remove the layer at this stage.
However, it is quite difficult to formulate an anti-halation layer so as to permit complete removal by water jets alone, insofar as one relies on a binder which is soluble in an aqueous alkaline solution, e.g., if one uses a binder readily soluble in alkaline solutions to permit the removal of the anti-halation layer by water jets alone, the strength of the layer becomes too low to withstand the strongly alkaline preliminary bath, and the layer tends to be stripped from the substrate and cause severe contamination of the preliminary bath.
A practically acceptable film resistance for an anti-halation layer in a strongly alkaline preliminary bath requires a binder with a relatively low solubility in an alkaline solution. Usually water jets fail to remove all of such an anti-halation layer, leaving a very thin film on the substrate.
Thus, it is conventional in processing to bring the photographic material, after it has been washed with water, into contact with a rotating cylinder which is covered with a soft material and is subjected to running water to rub off the remaining anti-halation layer.